Conclusions

- Changes in climate and land use have a larger direct impact than rising CO2 on global river runoff trends

Process-based simulations of global runoff using a terrestrial
biosphere model suggest that the observed significant increase in
global runoff in the 20th century was mainly a consequence of climate
change and widespread deforestation. We estimate that the secular rise
in atmospheric CO2 caused a small but significant decrease
in global runoff because of the antagonistic responses of leaf-level
processes and vegetation dynamics. On the basis of our findings, it
seems overoptimistic to assume that rising CO2 could cause
water savings in soil and thereby further promote vegetation
productivity at a scale large enough to affect continental runoff. The
results presented here not only provide insights for large-scale field
experiments but also highlight the importance of biosphere feedbacks on
the water balance of land surfaces (15, 16).
The roles of vegetation growth feedbacks and land-use change cannot be
ignored when projecting future changes in hydrologic processes and
climate.